Positive-electrode materials: methods for their preparation and use in lithium secondary batteries

ABSTRACT

A positive-electrode material for a lithium secondary battery. The material includes a lithium oxide compound or a complex oxide as reactive substance. The material also includes at least one type of carbon material, and optionally a binder. A first type of carbon material is provided as a coating on the reactive substance particles surface. A second type of carbon material is carbon black. And a third type of carbon material is a fibrous carbon material provided as a mixture of at least two types of fibrous carbon material different in fiber diameter and/or fiber length. Also, a method for preparing the material as well as lithium secondary batteries including the material.

CROSS REFERENCE TO RELATED APPLICATIONS

The present application is a divisional of U.S. application Ser. No.14/349,076, filed on Apr. 2, 2014, now U.S. Pat. No. 9,577,253, which isa U.S. national phase entry of International Application No.PCT/CA2012/050702, filed on Oct. 4, 2012, which claims the benefit ofCanadian Application No. 2,754,372, filed on Oct. 4, 2017. The entirecontents of each of U.S. application Ser. No. 14/349,076, InternationalApplication No. PCT/CA2012/050702, and Canadian Application No.2,754,372 are hereby incorporated herein by reference in their entirety.

FIELD OF THE INVENTION

The invention relates generally to positive-electrode materialscomprising a lithium oxide compound for use in lithium secondarybatteries. More specifically, the invention relates topositive-electrode materials comprising a lithium oxide compound and atleast two types of carbon material.

BACKGROUND OF THE INVENTION

Rechargeable batteries having high capacity when they are charged anddischarged at a high electricity current and that remain stable when thecharge and discharge process is repeated during a long period of timeare in high demand. Such batteries, which include lithium secondarybatteries, are used for example in electric vehicles, hybrid cars andthe like.

Typically, two categories of lithium secondary batteries are known. Afirst category in which the negative-electrode is formed by using amaterial capable of absorbing and discharging lithium ions, and a secondcategory in which the negative-electrode is formed by using metalliclithium. Lithium secondary batteries in the first category present atleast some advantages over those in the second category. For example inthe first category, safety of the battery is enhanced since there isless dendrite deposit and thus a short circuit between the electrodes isless likely to occur. Also, batteries in the second category generallyhave higher capacity and energy density.

In recent years, lithium secondary batteries in which anegative-electrode is formed by using a material capable of absorbingand discharging lithium ions have been in high demand. Extensiveresearch has been conducted aiming at improving the capacity of thebattery when it is charged and discharged at a high electric current andalso at improving its performance and life cycle, for up to several tensof thousands cycles. It has been found that the capacity of the batteryis improved by decreasing its electrical resistance. Also, the followinghave been found to be advantageous: (a) use of a positive-electrodematerial comprising a lithium metal oxide as the reactive substance anda negative-electrode material comprising carbon, leads to a highcapacity battery; (b) the total reactive surface of a substance in thebattery is increased by decreasing the mean size diameter of theparticles of the substance, or the reactive surface of the electrode isincreased by optimizing the design of the battery; (c) liquid diffusionresistance is decreased by making a separator thin.

When the mean size diameter of the particles of the reactive material issmall, the total reactive surface is increased. However, thisnecessitates an increase of the amount of binder used in the material.As a result, it becomes quite challenging to obtain a battery that has ahigh capacity. In addition, the positive-electrode andnegative-electrode materials have a tendency to peel or drop from themetal foil on which they are deposited. And since these materials areelectricity collectors, an internal short circuit inside the battery islikely to occur, resulting in a decrease in the voltage of the batteryand thermal runaway. Safety of the lithium secondary battery is thusimpaired.

Research has been conducted aiming at designing methods for increasingthe adherence of the positive-electrode and negative-electrode materialsto the metal foil on which they are deposited. Such methods include forexample altering the type of binder, as disclosed for example inJapanese laid-open patent application No. 5-226004.

Also, methods have been designed for allowing the lithium secondarybattery to have a high capacity when it is charged and discharged at ahigh electric current. For example, use of conductive carbon material todecrease the electrical resistance of the electrode has been disclosed;see for example Japanese laid-open patent applications No. 2005-19399,No. 2001-126733 and No. 2003-168429.

Although a suitable choice of binder used in the reactive materialallows for an increase of the capacity of the battery, this does notappear to have a positive effect on the improvement of the property ofthe battery whereby it has a high capacity when charged and dischargedat a high electric current, even when the electrical resistance of theelectrode is decreased.

When the battery is cyclically charged and discharged at a high electriccurrent, the positive-electrode and negative-electrode materials tend toexpand and contract. This leads to conductive paths of particles betweenthe positive and negative electrodes being impaired. As a result, a highelectric current cannot be circulated early on when the battery is used,and the battery thus has a short life span.

In recent years, lithium metal phosphate compounds such as olivine-typelithium iron phosphate as reactive substances of the positive-electrodein lithium secondary batteries have attracted attention; see for exampleJapanese laid-open patent applications No. 2000-509193 and No. 9-134724.Indeed, this reactive substance is safe and contributes to a decrease inthe cost of the battery since it is inexpensive. However, the substancehas a high electrical resistance and attempts to decrease the resistancehave been found quite challenging.

SUMMARY OF THE INVENTION

The present inventors have designed and prepared a material thatcomprises a lithium oxide compound and at least two types of carbonmaterial. The material according to the invention is used in lithiumsecondary batteries. The batteries using the material according to theinvention present improved electrical resistance properties.

The invention thus provides the following:

-   -   (1) A positive-electrode material for a lithium secondary        battery, comprising a lithium oxide compound as reactive        substance, at least one type of carbon material, and optionally        a binder, wherein:        -   a first type of carbon material is provided as a coating on            the reactive substance particles surface;        -   a second type of carbon material is carbon black; and        -   a third type of carbon material is a fibrous carbon material            provided as a mixture of at least two types of fibrous            carbon material different in fiber diameter and/or fiber            length.    -   (2) A positive-electrode material according to (1) above,        wherein the lithium oxide compound comprises a metal which is a        transition metal; preferably Fe, Mn, V, Ti, Mo, Nb, W, Zn or a        combination thereof; more preferably Fe.    -   (3) A positive-electrode material according to (1) above,        wherein the lithium oxide compound is a phosphate, an        oxyphosphate, a silicate, an oxysilicate, or a fluorophosphate;        preferably a phosphate.    -   (4) A positive-electrode material according to (1) above,        wherein the lithium oxide is LiFePO₄, LiMnPO₄, LiFeSiO₄, SiO,        SiO₂ or SiO_(x) (0≤x<2); preferably LiFePO₄.    -   (5) A positive-electrode material according to (1) above,        wherein the lithium oxide compound is a lithium phosphate,        preferably an olivine-type lithium iron phosphate.    -   (6) A positive-electrode material for a lithium secondary        battery, comprising a complex oxide compound as reactive        substance, at least one type of carbon material, and optionally        a binder, wherein:        -   a first type of carbon material is provided as a coating on            the reactive substance particles surface;        -   a second type of carbon material is carbon black; and        -   a third type of carbon material is a fibrous carbon material            provided as a mixture of at least two types of fibrous            carbon material different in fiber diameter and/or fiber            length, wherein the complex oxide compound is a complex            oxide of general formula A_(a)M_(m)Z_(z)O_(o)N_(n)F_(f),    -   wherein:        -   A represents an alkaline metal, preferably Li;        -   M represents a transition metal, and optionally at least one            non-transition metal, or a mixture thereof; preferably M is            Fe, Mn, V, Ti, Mo, Nb, W, Zn or a mixture thereof, and            optionally a non-transition metal which is Mg or Al; more            preferably, M is Fe;        -   Z represents a non-metallic element, preferably Z is P, S,            Se, As, Si, Ge, B or a mixture thereof;        -   N is a nitrogen atom;        -   F is a fluorine atom; and        -   a≥0, m≥0, z≥0, o>0, n≥0 and f≥0, a, m, o, n, f and z being            selected to ensure electro neutrality of the complex oxide.    -   (7) A positive-electrode material according to any one of (1)        to (6) above, wherein the carbon material coating is in graphene        or amorphous form, and bonds are formed between carbon atoms,        thereby facilitating electron conductivity.    -   (8) A positive-electrode material according to any one of (1)        to (6) above, wherein a thickness of the carbon material coating        layer is about 1 to 10 nm, preferably about 2 to 4 nm.    -   (9) A positive-electrode material according to any one of (1)        to (6) above, wherein the carbon black is conductive carbon        black; preferably acetylene black, ketjen black or a combination        thereof.    -   (10) A positive-electrode material according to any one of (1)        to (6) above, wherein the fibrous carbon material is a carbon        nanotube, a carbon nanofiber or a combination thereof.    -   (11) A positive-electrode material according to any one of (1)        to (6) above, wherein the at least two types of fibrous carbon        material are different in fiber diameter and fiber length.    -   (12) A positive-electrode material according to any one of (1)        to (6) above, wherein a first type of the fibrous carbon        material has fiber diameters of about 5 to 15 nm and fiber        lengths of about 1 to 3 μm, and a second type of the fibrous        material has fiber diameters of about 70 to 150 nm and fiber        lengths of about 5 to 10 μm.    -   (13) A positive-electrode material according to any one of (1)        to (6) above, wherein a combined amount of the carbon black and        the fibrous carbon material is not less than about 2 wt % for a        total amount of the material.    -   (14) A positive-electrode material according to any one of (1)        to (6) above, wherein a weight ratio of the carbon black and the        fibrous carbon material is about (2 to 8)/(1 to 3), preferably        6/2.    -   (15) A positive-electrode material according to any one of (1)        to (6) above, wherein the binder is a fluorine-containing resin,        preferably polytetrafluoroethylene, vinylidene polyfluoride, or        fluororubber; a thermoplastic resin, preferably polypropylene or        polyethylene; a dispersion-type resin, preferably styrene        butadiene rubber or polymer of acrylic acid; or a combination        thereof.    -   (16) A method of preparing a positive-electrode material for a        lithium secondary battery, comprising:        -   (a) providing a lithium oxide compound as reactive            substance;        -   (b) coating the reactive substance particles surface with a            carbon material; and        -   (c) mixing the coated reactive substance with carbon black,            a mixture of at least two types of fibrous carbon material            different in fiber diameter and/or fiber length, and            optionally a binder, wherein step (c) is performed by            compression shear impact-type particle-compositing            technique.    -   (17) A method of preparing a positive-electrode material for a        lithium secondary battery, comprising:        -   (a) providing a complex oxide compound as reactive            substance;        -   (b) coating the reactive substance particles surface with a            carbon material; and        -   (c) mixing the coated reactive substance with carbon black,            a mixture of at least two types of fibrous carbon material            different in fiber diameter and/or fiber length, and            optionally a binder, wherein step (c) is performed by            compression shear impact-type particle-compositing            technique, and    -   wherein:        -   A represents an alkaline metal, preferably Li;        -   M represents a transition metal, and optionally at least one            non-transition metal, or a mixture thereof; preferably M is            Fe, Mn, V, Ti, Mo, Nb, W, Zn or a mixture thereof, and            optionally a non-transition metal which is Mg or Al; more            preferably, M is Fe;        -   Z represents a non-metallic element, preferably Z is P, S,            Se, As, Si, Ge, B or a mixture thereof;        -   N is a nitrogen atom;        -   F is a fluorine atom; and        -   a≥0, m≥0, z≥0, o>0, n≥0 and f≥0, a, m, o, n, f and z being            selected to ensure electro neutrality of the complex oxide.    -   (18) A method according to (16) above or 17, further        comprising (d) calcining the mixture obtained in step (c).    -   (19) A method according to (16) or (17) above, wherein step (d)        is performed at a temperature of about 700 to 850° C.,        preferably about 720° C.    -   (20) A method according to (16) or (17) above, wherein step (d)        is performed during a period of time of about 0.5 to 2 hours,        preferably about 1 hour.    -   (21) A method according to (16) or (17) above, wherein step (d)        is performed under inert atmosphere, preferably argon        atmosphere.    -   (22) A method according to (16) above, wherein the lithium oxide        compound comprises a metal which is a transition metal;        preferably Fe, Mn, V, Ti, Mo, Nb, W, Zn or a combination        thereof; more preferably Fe.    -   (23) A method according to (16) above, wherein the lithium oxide        compound is a phosphate, an oxyphosphate, a silicate, an        oxysilicate, or a fluorophosphate; preferably a phosphate.    -   (24) A method according to (16) above, wherein the lithium oxide        is LiFePO4, LiMnPO4, LiFeSiO4, SiO, SiO2 or SiOx (0≤x<2);        preferably LiFePO4.    -   (25) A method according to (16) above, wherein the lithium oxide        compound is a lithium phosphate, preferably an olivine-type        lithium iron phosphate.    -   (26) A lithium secondary battery comprising the        electrode-positive material as defined in any one of (1) to (15)        above.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a pattern diagram of a positive-electrode materialfor a lithium secondary battery.

FIG. 2 shows a photograph of the surface of the positive-electrodematerial taken by a transmission-type electron microscope.

DESCRIPTION OF PREFERRED EMBODIMENTS

A lithium secondary battery is a secondary battery in which anelectrolyte is penetrated into a group of electrodes wound or layeredone upon another with a separator being interposed between apositive-electrode plate and a negative-electrode plate, or the group ofelectrodes is immersed in the electrolyte to repeatedly absorb andrelease lithium ions. A positive-electrode material is formed on thesurface of the positive-electrode plate, and a negative-electrodematerial is formed on the surface of the negative plate.

In accordance with the invention, the positive-electrode materialcomprises an active substance which is a lithium metal oxide compound,and at least two types of carbon material. In embodiments of theinvention, a first type of carbon material is in close contact with thesurface of particles in the reactive substance; this carbon material canbe in a graphene form, amorphous form or the like. A second type ofcarbon material in the positive-electrode material is a carbon black. Athird type of carbon material in the positive-electrode materialconsists of a mixture of two types of fibrous carbon material.

As can be seen in FIG. 1, a positive-electrode material 1 comprises anactive substance which is a lithium phosphate compound 2. The surface ofthe particles of the compound is coated with a carbon material 3. Thecoating layer can be in a graphene form, an amorphous form or the like.The thickness of the carbon material coating layer 3 can be severalnanometers. The lithium phosphate compound 2 is combined with the carbonblack 4 and the fibrous carbon material 5. The fibrous carbon material 5is a mixture of a first fibrous carbon material 5 a having a small fiberdiameter and a short fiber length and a second fibrous carbon material 5b having a large fiber diameter and a long fiber length. The firstfibrous carbon material 5 a is mainly connected to the surface ofparticles of the lithium phosphate compound 2, whereas the secondfibrous carbon material 5 b mainly connects particles of the lithiumphosphate compound 2.

As can be seen in FIG. 2, the first fibrous carbon material 5 a ismainly present on the surface of particles of the lithium phosphatecompound 2. And the second fibrous carbon material 5 b is presentbetween particles of the lithium phosphate compound 2.

Examples of lithium phosphate compounds that can be used for thepositive-electrode material of the present invention include LiFePO₄,LiCoPO₄, and LiMnPO₄. In embodiments of the invention, olivine-typelithium iron phosphate, LiFePO₄ is used. Indeed, LiFePO₄ presentsexcellent electrochemical properties and safety, and is inexpensive. Aswill be understood by a skilled person, any suitable lithium oxidecompound can be used.

The positive-electrode material 1 according to the invention can bedefined generally as being based on complex oxides of general formulaA_(a)M_(m)Z_(z)O_(o)N_(n)F_(f), wherein:

-   -   A represents one or more alkaline metals which can be for        example Li.    -   M represents one or more transition metals, and optionally at        least one non-transition metal, or a mixture thereof. For        example M can be Fe, Mn, V, Ti, Mo, Nb, W, Zn or mixtures        thereof, and optionally a non-transition metal, which can be Mg        or Al.    -   Z represents one or more non-metallic elements, wherein a≥0,        m≥0, z≥0, o>0, n≥0 and f≥0, the coefficients a, m, o, n, f and z        being selected to ensure electro neutrality. For example Z can        be P, S, Se, As, Si, Ge, B or a mixture thereof.    -   N is a nitrogen atom.    -   F is a fluorine atom.

For examples, complex oxides that can be used in the invention includephosphate, oxyphosphate, silicate, oxysilicate, and fluorophosphate.Preferably, the complex oxide is LiFePO₄, LiMnPO₄, LiFeSiO₄, SiO, SiO₂or SiO_(x) (0≤x<2). As will be understood by a skilled person, anysuitable complex oxide can be used.

The surface of particles of the olivine-type lithium iron phosphate iscoated with carbon material 3. At least one carbon material coatinglayer is formed on the particles surface. The coating layer can be in agraphene form, an amorphous form or the like. The carbon materialcoating layers are formed by methods generally known in the art. Suchmethods include for example: (a) dispersing conductive carbon black suchas acetylene black, Ketjen Black or graphite crystal in a solvent toform a slurry coating solution, dispersing particles of the olivine-typelithium iron phosphate in the coating solution, and thereafter dryingthe solvent; (b) applying an organic substance or an organic polymersolution to the surface of the particles of the olivine-type lithiumiron phosphate and thermally decomposing the organic substance or theorganic polymer in a reducing atmosphere; (c) an ion deposit method; and(d) forming a thin film on the surface of the particles of theolivine-type lithium iron phosphate by chemical evaporation method (CVD)and/or a physical evaporation method (PVD).

In accordance with the present invention, as used herein, “grapheneform” means one layer of a plain six-membered ring structure ofsp²-connected carbon atoms; and “amorphous form” means athree-dimensional six-membered ring structure. Electron conductivityoccurs due to bonds between carbon atoms, which are facilitated by thegraphene form or amorphous form of the carbon material coating. Also asused herein the term “about” means plus or minus 10% of the statedvalue.

The carbon material coating 3 is in close contact with the surface ofparticles of the active substance 2. The thickness of the carbonmaterial coating layer is about 1 to 10 nm, preferably about 2 to 5 nm.When the thickness of the coating layer is less than 1 nm, electronconductivity through bonds between carbon atoms is limited. When thethickness of the coating layer is greater than about 10 nm, diffusion oflithium ions on the surface of particles of the active substancedecreases, and output property of the battery deteriorates.

In accordance with the invention, a second type of carbon material inthe positive-electrode material is carbon black. In embodiments of theinvention, carbon black can be for example conductive carbon black 4.Such conductive carbon black includes for example acetylene black andKetjen black. As will be understood by a skilled person, any suitablecarbon black material can be used.

In accordance with the invention, a third type of carbon material in thepositive-electrode material is fibrous carbon material 5. The fibrouscarbon material can be a carbon nanotube or a carbon nanofiber. As usedherein, “carbon nanotube” means a tube consisting of a single-walledring, and “carbon nanofiber” means a tube consisting of a multi-walledring.

In embodiments of the invention, the fibrous carbon material 5 is amixture of the two types of fibrous carbon material 5 a, 5 b. The twotypes can be different in at least one of fiber diameters and fiberlengths. That is, it is possible to use (a) fibrous carbon materialsdifferent in both fiber diameters and the fiber lengths, (b) fibrouscarbon materials equal in fiber diameters but different in fiberlengths, and (c) fibrous carbon materials different in fiber diametersbut equal in fiber lengths. Preferably, fibrous carbon materialdifferent in both fiber diameters and fiber lengths are used.

The diameter of one type of fibrous carbon material can be about 5 to 15nm, while the diameter of the other type of fibrous carbon material isabout 70 to 150 nm. Preferably, the diameter of one type of the fibrouscarbon material is about 10 nm, while the diameter of the other type offibrous carbon material is about 100 nm.

The fiber length of the fibrous carbon material having diameter of about5 to 15 nm can be about 1 to 3 μm, preferably about 3 μm. The fiberlength of the fibrous carbon material having the diameter of about 70 to150 nm can be about 5 to 10 μm, preferably about 5 μm. That is, in thepresent invention, it is preferable to use a mixture of two types offibrous carbon materials, a first type having a small fiber diameter anda short fiber length and a second type having a large fiber diameter anda long fiber length.

In the positive-electrode material of the present invention, a totalcontent of the carbon black and a total content of the fibrous carbonmaterial is not less than about 2 wt %, preferably about 2 to 10 wt % ofa total amount of the lithium phosphate compound coated with carbonmaterial, the carbon black, and the fibrous carbon material.

In embodiments of the invention, a weight ratio of carbon black andfibrous carbon material (carbon black/fibrous carbon material) is about(2 to 8)/(1 to 3), preferably about 6/2.

The positive-electrode material of the present invention is prepared bymixing together the lithium phosphate compound 2 coated with carbonmaterial 3, the carbon black 4, and the fibrous carbon material 5. Thisis performed a method known as compression shear impact-typeparticle-compositing method. As will be understood by a skilled personany suitable mixing technique can be used.

In the compression shear impact-type particle-compositing method,powders applied to an inner wall of a rotary container by a centrifugalforce are mixed between the rotary container and a press head, having aradius of curvature different from that of the rotary container, whichis fixed to the inside of the rotary container, while a strongcompression shearing force is being applied to the powders. A mixingapparatus used in this method can be for example a Nobilta™ machine or aMechanofusion™ mixing machine (Hosokawa Micron Corporation).

Following the mixing step as described above, the mixture is calcined,thereby causing the surfaces of particles of the reactive substance tocomposite with one another due to the bond between the carbon atoms. Asa result, electron conductivity between the particles surfaces of thematerial is greatly improved. In embodiments of the invention, themixture can be calcined at a temperature of about 700 to 850° C., underan inert atmosphere, for about 0.5 to 2 hours. Preferably, the mixtureis calcined at a temperature of about 720° C., under inert atmosphere,for about 1 hour. The inert atmosphere can be for example an argonatmosphere.

In accordance with the invention, binders can be used in thepositive-electrode material. Suitable binders are selected such thatthey are physically and chemically stable under the conditions insidethe battery. Such binders include for example fluorine-containing resinsuch as polytetrafluoroethylene, vinylidene polyfluoride, andfluororubber; thermoplastic resin such as polypropylene andpolyethylene; and dispersion-type resin such as styrene butadiene rubberand polymers of acrylic acid.

In accordance with the invention, a separator is used in the lithiumsecondary battery together with the positive-electrode material. Theseparator electrically insulates the positive-electrode and thenegative-electrode from one another. In embodiments of the invention, afilm made of a synthetic resin, a fiber, or an inorganic fiber can beused as separator. Preferably, a polyethylene film or a polypropylenefilm; woven cloth and unwoven cloth made of these resins; and glassfibers and cellulose fibers are used.

In accordance with the invention, electrolytes of the lithium secondarybattery in which the group of electrodes is immersed include for examplenon-aqueous electrolytes containing lithium salts or ion-conductingpolymers. In embodiments of the invention, non-aqueous solvents in thenon-aqueous electrolytes containing the lithium salts, ethylenecarbonate (EC), propylene carbonate (PC), diethyl carbonate (DEC),dimethyl carbonate (DMC), and methyl ethyl carbonate (MEC) can be used.As will be understood by a skilled person, any suitable electrolyte canbe used.

In accordance with the invention, lithium salts dissolved in theelectrolyte. In embodiments of the invention, lithium salts dissolved innon-aqueous electrolytes can be for example lithium hexafluorophosphate(LiPF₆), lithium borotetrafluoride (LiBF₄), and lithiumtrifluoromethanesulfonate (LiSO₃CF₄). As will be understood by a skilledperson, any suitable lithium salt can be used.

In accordance with the invention, the positive-electrode material forthe lithium secondary battery can be formed by layering the material onthe surface of the positive-electrode plate serving as an electricitycollector. The positive-electrode plate can be for example metal thinfilms. In embodiments of the invention, an aluminum foil can be used asthe positive-electrode plate. And for the negative-electrode plate, acopper foil or a plate made of carbon material can be used.

EXAMPLES

The positive-electrode material of the present invention for lithiumsecondary batteries is described in detail below by way of examples andcomparative examples. However, it will be understood that the presentinvention is not limited to those examples.

Examples 1, 2 and Comparative Examples 1 through 5

The olivine-type lithium iron phosphate (LiFePO₄) having a mean particlediameter of about 0.5 to 2 μm was used as the active substance of thepositive-electrode. The olivine-type lithium iron phosphate was coatedwith the carbon material forming a coating layer having a thickness ofabout 3 nm, using an evaporation method in which carbonized gas wasused.

Carbon nanotube and acetylene black having configurations and amountsshown in Table 1 were added to the active substance and mixed togetherusing the Nobilta mixing machine (Hosokawa Micron Corporation) bycompression shear impact-type particle-compositing method. The mixingratio between the carbon nanotube and acetylene black (acetyleneblack/carbon nanotube) was about 8/2 in a mass ratio. The mixing methodcarried out by using the Nobilta mixing machine is shown as “mixing” inthe column of “electrical conductive material addition method” in Table1.

Six parts by mass of vinylidene polyfluoride was added to 97 parts bymass of the mixture as a binder. N-methylpyrrolidone was added as adispersion solvent to the mixture. The components were kneaded toprepare a positive-electrode slurry as the active substance of thepositive-electrode for the lithium secondary battery.

An aluminum foil having a thickness of 20 μm and a width of 150 mm wasprepared. The positive-electrode slurry was applied to both surfaces ofthe aluminum foil and dried. Thereafter the aluminum foil to which thepositive-electrode slurry was applied was pressed and cut to obtain apositive-electrode plate for the lithium secondary battery. The totalthickness of the positive-electrode plate after the positive-electrodeslurry was applied to both surfaces of the aluminum foil and thealuminum foil was dried and pressed was 160 μm.

A laminate battery of 20 mAh was produced by using thepositive-electrode plate. A negative pole made of graphite material wasused as the electrode opposite to the positive-electrode plate. Unwovencloth made of olefin fiber was used as a separator for electricallyinsulating the positive-electrode plate and the negative-electrode platefrom each other. An electrolyte used was composed of lithiumhexafluorophosphate (LiPF₆) dissolved at 1 mol/l in a solution in whichethylene carbonate (EC) and methyl carbonate (MEC) were mixed with eachother at a volume ratio of 30:70.

To examine the discharge performance of the batteries, a discharge testand a life cycle test were conducted.

Discharge Test

After the battery was charged, charge/discharge efficiency became nearly100% was confirmed, a discharged capacity was measured when each batterywas discharged up to 2.0 V at a constant electric current of 4 mA.Thereafter a discharged capacity was measured at electric current of 200mA. The discharged capacity at the electric current of 200 mA wasexpressed as the ratio with respect to the discharged capacity at theelectric current of 4 mA. The discharge performance is shown in Table 1as evaluation of discharge test (%).

Life Cycle Test

Each battery was charged (finished at electric current of 1 mA) at aconstant electric current and a constant voltage of 4.0 V (limitedelectric current of 60 mA). Each battery was discharged up to 2.0 V at aconstant electric current of 60 mA. Each charging and dischargingoperation was suspended for 10 minutes. A series of charge, suspension,and discharge was set as one cycle. The ratio of a discharged capacityat the 200th cycle to that at the first cycle was calculated as thedischarge capacity maintenance ratio. The discharge capacity maintenanceratio is shown in Table 1 as a life cycle test (%).

Examples 3 through 5

Olivine-type lithium iron phosphate (LiFePO₄) having a secondaryparticle diameter of 0.5 to 2 μm was used as the active substance of thepositive-electrode. The olivine-type lithium iron phosphate was coatedwith the carbon material and a coating layer having a thickness of about3 nm was formed. This was done by using an evaporation method in whichcarbonized gas was used.

Carbon nanotube and acetylene black having configurations and amountsshown in Table 1 were added to the active substance of the positiveelectrode and mixed together in the Nobilta mixing machine (HosokawaMicron Corporation) by compression shear impact-typeparticle-compositing method.

The mixture was calcined at 700 to 800° C. for one hour under inertatmosphere. The case in which the mixture was calcined after theabove-described components were mixed with each other using the Nobiltamixing machine is shown as “compositing” in the column of “electricalconductive material addition method” in Table 1.

By using the calcined mixture, a positive-electrode slurry was producedby a method similar to that of Example 1. By using thepositive-electrode slurry, a laminate battery of 20 mAh was produced ina method similar to that of Example 1. Table 1 shows results ofevaluation made in a method similar to that of Example 1.

TABLE 1 Electrical Total amount conductive of electrical Evaluationmaterial Carbon nanotube conductive of Life Type of addition DiameterLength material discharge cycle battery method (nm) (μm) (mass %) test(%) test (%) Example 1 Mixing 10 10 3 86 83 100 10 Example 2 Mixing 10 23 91 89 100 10 Example 3 Compositing 10 10 3 92 91 100 10 Example 4Compositing 10 2 3 95 96 100 10 Example 5 Compositing 10 2 8 99 98 10010 Comparative Mixing 200 15 1 22 19 example 1 Comparative Mixing 200 71 31 33 example 2 Comparative Mixing 3 15 1 53 48 example 3 ComparativeMixing 3 7 1 60 55 example 4 Comparative Mixing 3 7 3 82 72 example 5

The results of the evaluation of the discharge test shown in Table 1indicate that the lithium secondary batteries using thepositive-electrode materials of the Examples 1 through 5 have improvedproperties over batteries using the positive-electrode materials of thecomparative examples 1 through 5.

For example, in Example 1, a mixture of two types of carbon nanotubessmall and large in the fiber diameters was used. In Comparative Example1, only one type of carbon nanotube large in its fiber diameter and longin its fiber length was used. In Comparative Example 2, only one type ofcarbon nanotube large in its fiber diameter and short in its fiberlength was used. In Comparative Example 3, only one type of carbonnanotube small in its fiber diameter and long in its fiber length wasused. In each of Comparative Examples 4 and 5, only one type of carbonnanotube small in the fiber diameter thereof and short in the fiberlength thereof was used.

The results of the evaluation of Comparative Examples 1 through 5indicate that the battery has excellent performance when it contains alarge amount of the carbon nanotube small in its fiber diameter and thecarbon nanotube large in its fiber diameter. It is considered that thisis attributed to the conductivity of the olivine-type lithium ironphosphate which is active substance particles and excellence in aconductive network of particles. As in the case of the battery ofComparative Example 5, the battery containing a large absolute amount ofelectrical conductive materials has improved properties.

But as shown in Examples 1 through 5, when a mixture of two types ofcarbon nanotubes small and large in the fiber diameters thereof wasused, excellent results were obtained in the discharge test and thecycle life test. This is attributed to an improved electron conductivityowing to the disposition of the mixed carbon nanotubes on the surface ofthe active substance particles and between the active substanceparticles.

It is more favorable to vary the fiber length of the carbon nanotubesand form a network between the carbon nanotubes and particles disposedon the periphery thereof.

The carbon nanotubes of Examples 3 through 5 bond with the surfaces ofthe particles of the positive-electrode active substance bycompositeness not by contact resistance between carbon particles but bybond between carbon atoms (C—C). This is the reason why the batteries ofExamples 3 through 5 have an improved electron conductivity.

Acetylene black is present on the surfaces of the particles of theactive substance of the positive-electrode and carbon nanotubes.Therefore the acetylene black bonds not only with the carbon coating thesurfaces of the particles of the active substance of thepositive-electrode, but also with the carbon nanotubes. It is consideredthat all particles are coated with two layers of the electricalconductive carbon material.

The results of the evaluation made in the life cycle test in which eachbattery was charge and discharged at a high current had a tendencysimilar to that of the results of the evaluation made in the dischargetest. The discharge test and the life cycle test prove that the lithiumsecondary battery using the positive-electrode material of the presentinvention present improved properties.

Although the present invention has been described hereinabove by way ofspecific embodiments thereof, it can be modified, without departing fromthe spirit and nature of the subject of the invention as defined in theappended claims.

The present invention refers to a number of documents, the content ofwhich is herein incorporated by reference in their entirety.

The invention claimed is:
 1. A method of preparing a positive-electrodematerial for a lithium secondary battery, comprising: (a) providing acomplex oxide compound as reactive substance; (b) coating a surface ofparticles of the reactive substance with a carbon material; and (c)mixing the coated reactive substance with (i) carbon black, (ii) amixture of at least a first fibrous carbon material and a second fibrouscarbon material, the second fibrous carbon material being different infiber diameter and/or fiber length from the first fibrous carbonmaterial, and (iii) optionally a binder, wherein step (c) is performedby compression shear impact particle-compositing technique, wherein thecomplex oxide compound is a complex oxide of general formulaA_(a)M_(m)Z_(z)O_(o)N_(n)F_(f), and wherein: A represents an alkalinemetal; M represents a transition metal, and optionally at least onenon-transition metal, or a mixture thereof; Z represents a non-metallicelement; N is a nitrogen atom; F is a fluorine atom; and a≥0, m≥0, z≥0,o>0, n≥0 and f≥0, a, m, o, n, f and z being selected to ensure electroneutrality of the complex oxide, wherein the first fibrous carbonmaterial has a fiber diameter of about 5 to 15 nm and the second fibrouscarbon material has a fiber diameter of about 70 to 150 nm, wherein thefirst fibrous carbon material has a fiber length of about 1 to 3 μm andthe second fibrous carbon material has a fiber length of about 5 to 10μm.
 2. A method according to claim 1, further comprising (d) calcining aa mixture obtained in step (c).
 3. A method according to claim 1,wherein step (d) is performed at a temperature of about 700 to 850° C.4. A method according to claim 1, wherein step (d) is performed during aperiod of time of about 0.5 to 2 hours.
 5. A method according to claim1, wherein step (d) is performed under inert atmosphere.
 6. A methodaccording to claim 1, wherein: A is Li; M is Fe, Mn, V, Ti, Mo, Nb, W,Zn or a mixture thereof; and Z is P, S, Se, As, Si, Ge, B or a mixturethereof.